In general, pigments have been used for coating materials, printing ink and writing ink. Most of the pigments were solvent type. However, in consideration of bad influence due to the solvents for human bodies, saving of resources and countermeasure for environment pollution, recently solvent type coating materials and solvent type ink are gradually transformed into water-based type coating materials and water-based type ink, respectively. Although various physical properties such as light resistance, water resistance, alkali resistance and solvent resistance are demanded in the field of coating materials and ink, the water-based type coating materials and the water-based type ink have many problems for physical properties.
For instance, in the field of coating materials, when a surface active agent is used as a dispersant for a pigment, coating film property such as water resistance of coating materials is considerably lowered. Accordingly, in general, an alkali-neutralized type aqueous resin has been used as a dispersant. However, even if the aqueous resin was used, coating film properties such as alkali resistance and solvent resistance were poorer than those of solvent type coating materials. The properties of coating materials in which the aqueous resin was used were insufficient.
Similarly, as to writing ink, various physical properties such as water resistance, alkali resistance and solvent resistance are demanded in addition to writing property. However, these physical properties including stability during the passage of time are, indeed, insufficient.
In order to overcome the above defects, for instance, the following compositions are proposed. A water-based coating composition of which solvent resistance is improved and in which aggregation of a pigment is prevented during preservation by dispersing the pigment with cross-linked resin particles having an ionic group (Japanese Unexamined Patent Publication No. 22367/1990) A coating composition containing a pigment for printing ink, which is prepared by kneading a water-based cross-linked resin dispersion prepared by reacting isocyanate group (terminal group) in an urethane polymer and cross-linked, and a pigment (Japanese Unexamined Patent Publication No. 138518/1995) A water-based coating composition of which coloring and gloss are improved by carrying out co-precipitate using polyurethane as anionic cross-linked resin particles under the acidic condition and redispersion (Japanese Unexamined Patent Publication No. 170039/1996).
However, in the above coating compositions, a cross-linked resin is used as a dispersant. Molecular chain of the cross-linked resin is long, and it is difficult to adsorb the cross-linked resin around a pigment. As a result, when the pigment is dispersed with the cross-linked resin, it is difficult to sufficiently disperse particles, and there is bad influence for stability during the passage of time. Accordingly, the above coating compositions are insufficient.
For instance, the following various methods are proposed. A method for preventing elution of a pigment from a printing ink coating layer even under the condition at higher humidity by interacting the specific carboxylic acid as a dispersant for a pigment with an aliphatic polyol and an isocyanate (Japanese Unexamined Patent Publication No. 134635/1980) A process for preparing water-based writing ink of which light resistance, water resistance and solvent resistance are improved, and in which dispersing stability of a pigment is improved by using a polymer mainly comprising a polyester having both hydrophilic parts and lipophilic parts, as a dispersant (Japanese Unexamined Patent Publication No. 26070/1985).
However, improvement of water resistance, alkali resistance and solvent resistance of pigment ink prepared in accordance with the above methods is insufficient, and dispersion of the pigment is insufficient. As a result, stability during the passage of time is poor, so that precipitate is generated. Furthermore, light resistance is poor. Accordingly, quality of the above pigment ink is insufficient.
A water-based dispersion prepared by wrapping a pigment in a resin to form a capsule is disclosed in Japanese Unexamined Patent Publication No. 221137/1991 and Japanese Unexamined Patent Publication No. 151342/1997. There are characteristics that dispersion step is saved during preparation of the water-based dispersion, and that the water-based dispersion is excellent in water resistance and redispersing property. However, phase transition and emulsification step from an organic phase is necessitated when the above water-based dispersion is prepared. In addition, because a solvent in the organic phase must be removed and collected, steps for preparation are complicated.
The present invention has been completed in consideration of the above prior arts. An object of the present invention is to provide a water-based pigment dispersion showing excellent light resistance, water resistance, alkali resistance and solvent resistance, and in addition, showing excellent stability during the passage of time, which can be suitably used for coating materials, paper coating, textile printing, writing ink, printing ink, ink-jet printing ink, color filters, cosmetic materials and electrophotographic toner, and to provide a process for easily preparing the water-based pigment dispersion and a water-based ink containing the water-based pigment dispersion.